Dark Matter Halo Mergers I: Dependence on Environment & Redshift Evolution

TL;DR

This study uses a large cosmological simulation to analyze how environment and redshift influence the rate of dark matter halo mergers, revealing environmental effects and dynamical processes affecting galaxy evolution.

Contribution

It provides new insights into the environmental dependence of halo merger rates and the role of tidal stripping and subhalo interactions in merger dynamics.

Findings

Major mergers are rare between subhalos due to tidal stripping.

Merger timescales are longer without other subhalos, but shorter with additional subhalos.

Environmental factors cause slight suppression or enhancement of merger rates in different contexts.

Abstract

This paper presents a study of the specific merger rate as a function of group membership, local environment, and redshift in a very large, $500h^{-1} Mpc$, cosmological N-body simulation, the \textit{Millennium Simulation}. The goal is to provide environmental diagnostics of major merger populations in order to test simulations against observations and provide further constraints on major merger driven galaxy evolution scenarios. A halo sample is defined using the maximum circular velocity, which is both well defined for subhalos and closely correlated with galaxy luminosity. Subhalos, including the precursors of major mergers, are severely tidally stripped. Major mergers between subhalos are therefore extremely rare. Tidal stripping also suppresses dynamical friction, resulting in long major merger time scales when the more massive halo does not host other subhalos. In contrast, when other subhalos are present major merger time scales are several times shorter. This enhancement is likely due to inelastic unbound collisions between subhalos. Following these results, we predict that major mergers in group environments are dominated by mergers involving the central galaxy, that the specific merger rate is suppressed in groups, and that the frequency of fainter companions is enhanced for mergers and their remnants. We also observe an `assembly bias' in the major merger rate in that mergers of galaxy-like halos are slightly suppressed in overdense environments while mergers of group-like halos are slightly enhanced. A dynamical explanation for this trend is advanced which calls on both tidal effects and interactions between bound halos beyond the virial radii of locally dynamically dominant halos.